1. Field of the Invention
The present invention relates to a manufacturing method of a semiconductor device, and more specifically a method for forming a trench element isolator region.
2. Description of the Prior Art
For finer configuration and enhancement of a manufacturing speed of a semiconductor element, it is necessary to narrow width of an element isolator. Though an LOCOS method has conventionally been used in general as a method to form the element isolator region, this method cannot sufficiently cope with a demand for such configuration. As a method substituting for the LOCOS method, an STI (Shallow Trench Isolation) method has recently been attracting attention.
The conventional STI method forms the element isolator by laminating a thin pad oxide film and a nitride film on a semiconductor substrate such as a silicon substrate, forming a resist mask having a region open for isolating the element by a photolithography method, forming a trench by anisotropically etching the nitride film, the pad oxide film and the semiconductor substrate using the resist mask as an etching mask, depositing an insulating material over an entire surface after removing the resist mask, polishing the semiconductor substrate by a Chemical Mechanical Polishing (CMP) method using the nitride film as a stopper and embedding an insulating material into the trench.
In the case where a pattern of the resist mask cannot be formed in a desired shape, this method may require releasing the resist mask and forming a resist mask once again (reconstruction). However, this method poses a problem that an acid solution which is used as a resist mask releasing solution roughens a surface of the nitride film, thereby changing an optical characteristic on the surface of the nitride film and degrading an efficiency of the reconstruction. That is, the method allows thickness of the nitride film to be varied and may make it impossible to form the pattern in the desired shape under influence due to a standing wave, thereby varying film thickness for each reconstruction and making it extremely difficult to form the pattern in the desired shape.
In order to solve such a problem in the reconstruction, it has conventionally been proposed to deposit a silicon oxide film on the nitride film. Description will be made of this conventional technique with reference to FIGS. 5(A) through 5(D) and FIGS. 6(A) through 6(C).
FIGS. 5(A) through 5(D) are sectional views illustrating steps of forming a trench element isolator by a conventional method. First, a thin pad oxide film 2 is formed on a semiconductor substrate such as an Si substrate 1 as shown in FIG. 5(A) by a thermal oxidation method or the like and a silicone nitride film 3 having desired thickness is successively formed by an LPCVD method or the like. Furthermore, a silicon oxide film 4xe2x80x2 having thickness on the order of 1000 to 2000 xc3x85 is formed on the silicon nitride film 3 by the LPCVD method or the like.
Successively, a resist mask 5 is formed by applying a resist and carrying out an ordinary photolitho process so as to open a portion which is to form an element isolator, and an opening 6 is formed as shown in FIG. 5(B) by consecutively dry etching the oxide film 4xe2x80x2, the nitride film 3 and the pad oxide film 2 using the resist mask 5 as an etching mask until a surface of the Si substrate 1 is exposed. Successively, the resist mask 5 is released and a trench 7 on the order of 2000 to 4000 xc3x85 is formed by anisotropically etching, dry etching in particular, the exposed surface of the Si substrate 1 using the oxide film 4xe2x80x2 as an etching mask (FIG. 5(C)).
The oxide film 4xe2x80x2 must be formed thick as described above since the oxide film 4xe2x80x2 is also etched and thinned when the Si substrate 1 is etched.
The resist mask 5 is released before etching the Si substrate since etching residues have been accumulated on the resist mask.
That is, the etching residues (hereinafter referred to as xe2x80x9cetching residues deriving from insulating materialsxe2x80x9d) have been produced due to a reaction between an oxide film etching gas and the oxide film, a reaction between a nitride film etching gas and the nitride film and reactions between these etching gases and a resist material while etching the oxide film 4xe2x80x2, the nitride film 3 and the pad oxide film 2. Furthermore, etching residues such as a product formed by a reaction between Si and an etching gas (hereinafter referred to as xe2x80x9cresidues deriving from Sixe2x80x9d) are formed when the Si substrate is etched using the resist mask 5 as the etching mask.
These etching residues should be removed before an oxide film is subsequently embedded into the trench. The residues deriving from the insulating materials can be removed by a treatment with an acid solution, for example, a resist releasing solution, a treatment with O2 plasma, whereas the etching residues deriving from Si can hardly be removed.
On the other hand, treatment only with the acid solution makes it difficult to remove the residues deriving from the insulating materials and all the etching residues cannot be removed at the same time. Accordingly, the method according to the present invention is configured by releasing the resist mask 5 before etching the Si substrate for removing the residues deriving from the insulating materials, then etch the Si substrate using the oxide film 4xe2x80x2 as the etching mask and thereafter remove the etching residues deriving from Si.
After these steps, an oxide film is embedded into the trench 7 which has been formed. Since the silicon substrate has been damaged by the anisotropic etching which has been carried out to form the trench, a thermally oxidized film 8 is formed in this step to correct the damage. (FIG. 5(D)).
Successively, a thick CVD oxide film 9 is deposited over an entire surface by a CVD method such as an HDPCVD (High Density Plasma Chemical Vapor Deposition) method (FIG. 6(A)) and the CVD oxide film 9 is polished by the Chemical Mechanical Polishing (CMP) method using the nitride film 3 as a stopper (FIG. 6(B)). Finally, the nitride film 3 is removed with heated phosphoric acid and the pad oxide film 2 is removed with hydrofluoric acid solution, thereby forming a trench element isolator as shown in FIG. 6(C).
Since the oxide film 4xe2x80x2 formed on the nitride film 3 is thinned by etching the Si substrate 1 as described above, the oxide film 4xe2x80x2 must be formed thick. A reason is that the oxide film 4xe2x80x2 which is formed thin is also etched and disappears (4xe2x80x3), thereby exposing the nitride film 3 as shown in FIG. 7 when the Si substrate is etched. If the trench is etched continuously in a condition where the nitride film 3 is exposed, a pole like residue 12 will be formed on a bottom of the trench by a product formed by a reaction between an etching gas and the nitride film. Since production of such a residue poses a problem of making it impossible to form the trench in the desired form, the oxide film 4xe2x80x2 is formed thick enough to prevent such a residue from being produced.
When an opening is formed by etching the thick oxide film 4, however, there is posed a problem that a width variation of the opening is larger in proportion to film thickness. Under the present circumstances where semiconductor devices are configured finer and finer, there is a tendency to narrow variation ranges allowable for opening widths and such a thick oxide film is going not to cope with a narrow variation range of an opening width.
An object of the present invention is to meet the demand for a finer semiconductor device by thinning an oxide film which must be conventionally formed thick on a nitride film.
A manufacturing method of a semiconductor device according to the present invention comprises a step of annealing a CVD oxide film which is formed on a nitride film by a conventional method, thereby densifying the CVD oxide. As a result, the densified oxide film is etched hardly and thinned less at a step of etching a semiconductor substrate such as a silicon substrate. That is, the manufacturing method according to the present invention which densities the CVD oxide film eliminates a conventional necessity to form a thick oxide film on a nitride film and prevents an opening width from being varied.
That is, the manufacturing method of a semiconductor device according to the present invention is a manufacturing method comprising a step of forming a silicon nitride film on a semiconductor substrate and forming a CVD silicon oxide film on the silicon nitride film, a step of patterning the silicon nitride film and the CVD silicon oxide film with a resist mask, a step of forming a trench by etching the semiconductor substrate using the patterned silicon nitride film and the patterned CVD silicon oxide film as an etching mask after releasing the resist mask, and a step of embedding an insulating material into the trench and flattening the embedded insulating material using the silicon nitride film as a stopper, wherein the manufacturing method comprises a step of annealing the semiconductor substrate after the step of forming the CVD silicon oxide film and before the step of etching the semiconductor substrate.